In Partial Wave Spectroscopy (PWS), a focused wave of broadband, low-spatially-coherent light illuminates a sample, and an image formed by back-scattered photons is acquired in the far field. A spectrum of the back-scattered light intensity is recorded for each pixel of the image. PWS combines certain aspects of microscopy and the spectroscopy of light elastically scattered by cells. However, unlike conventional microscopy, in which an image is formed by integrating the reflected or transmitted intensity over a broad spectrum, PWS measures spectral fluctuations in the back-scattering spectra. Unlike elastic scattering spectroscopy, where a signal is formed by the far-field interference of all waves propagating within a scattering particle, the spectrum analyzed in PWS is formed by a subset of these waves (a.k.a., “partial waves”).
When applied to a sample comprising cells, PWS virtually divides a cell into a collection of parallel channels each with a diffraction-limited transverse size, detects back-scattered waves propagating along 1D trajectories within these channels, and quantifies the statistical properties of the nanoarchitecture of a cell by the analysis of the fluctuating part of the (normalized) reflected intensity R(λ, x, y), where λ is the wavelength of light, and x and y are the spatial coordinates of a particular channel. By allowing analysis of the nanoarchitecture of cells in a sample, PWS is capable of detecting nanoarchitectural alterations in cells that are otherwise histologically indistinguishable.
First generation PWS instruments perform a single measurement in 3-4 minutes per cell. When combined with time for position selection and focusing, a sample of at least 30 cells, a number commonly required for diagnosis, requires 4-5 hours to acquire the necessary data for a single patient slide. These performance numbers do not allow for the high volume of patient slides required to complete the measurements required for both clinical and complex biological studies.